Method for controlling the feed composition to a process for polymerizing isobutylene

ABSTRACT

A process for processing a cracked feed stream so as to provide a feed stream, having a requisite concentration of isobutylene and butene-1, to a polymerization unit for polymerization of isobutylene.

The present invention relates to a method for controlling the composition of a feed stream to a process for producing polymers of isobutylene.

In order to produce polymers of isobutylene having certain desired physical characteristics, the feed to a polymerization system must contain appropriate concentrations of monomer. Particularly, it has been determined that the concentration of isobutylene in the polymerization system feed must be at least about 20 volume percent of the total volume of feed. In certain special situations, it can be important to have an exceptionally high concentration of isobutylene in the polymerization system feed generally exceeding about 40 volume percent. It has further been determined that an excessively high concentration of butene-1 as represented by the volumetric ratio of butene-1 to isobutylene in the polymerization system feed is undesirable and should not exceed about 0.30:1; otherwise, an undesirable polyisobutylene product will be produced by the polymerization process.

It is desirable to use a cracked feedstock as a feed to an isobutylene polymerization process; but, due to the concentrations of the light olefins, such cracked feedstocks cannot, without further processing, be processed as a polymerization feed.

Thus, one object of the invention is to provide a method of processing a cracked feedstock so as to provide a feedstock suitable as a feed to a polymerization system for polymerizing isobutylene.

Another object of the invention is to provide a method for controlling the composition of a feed stream to a polymerization system for polymerizing isobutylene.

Therefore, the inventive process provides for controlling the composition of a feed stream to a polymerization unit. A cracked feed is passed to a hydroisomerization unit which produces a hydroisomerate stream. The hydroisomerate stream is passed to a distillation zone whereby it is separated into a first overhead stream comprising isobutylene and a first bottoms stream comprising butene-2. The first overhead stream is utilized as the feed stream to the polymerization unit for producing polyisobutylene and an effluent stream. The first bottoms stream from the distillation zone is mixed with the effluent stream to form a mixture that is passed to an extractive distillation unit whereby the mixture undergoes an extractive distillation utilizing a suitable extractive solvent to separate paraffins and olefins by providing a second overhead stream comprising at least one paraffin compound and a third overhead stream comprising at least one olefin compound. The third overhead stream is passed to an isomerization unit to thereby produce an isomerate stream. The isomerate stream can be recycled as a feed to the hydroisomerization unit.

In the accompanying drawing, FIG. 1 provides a schematic representation of the inventive process.

Other objects and advantages of the invention will be apparent from the foregoing detailed description of the invention and the appended claims.

The inventive process utilizes a novel arrangement of subprocesses and separation means to give an overall integrated process system that provides a feed stream, having a desired composition of linear and non-linear olefin compounds, to a polymerization unit for polymerizing isobutylene to form a polyisobutylene product. Thus, in accordance with the present invention, an olefin feed stream is passed or charged to a hydroisomerization process system or unit whereby a hydroisomerate stream is produced and which can be used as a feed stream to a polymerization unit.

The olefin feed stream can contain olefin compounds. Generally, the olefin feed stream includes olefin compounds having 3 to 5 carbon atoms, preferably, those olefin compounds having 4 carbon atoms. Thus, the olefin feed stream can comprise butenes which can include butene-1, butene-2, and isobutylene. Depending upon its source, the olefin feed stream can contain diolefin compounds which include primarily butadiene.

Ultimately, the olefin feed stream will originate from a cracked feed stream. The cracked feed stream can be from any source which suitably provides for a stream containing a mixture of paraffins and olefins or a mixture of olefins. It is preferred for the cracked feed to comprise or consist essentially of such olefins or paraffins having four carbon atoms per molecule and further containing a concentration of isobutylene. Suitable sources of a cracked feed include the C4 fraction of the product from a fluidized catalytic cracking unit for cracking a gas oil petroleum fraction and the product from a thermal cracking unit, or steam cracker, for cracking light hydrocarbons and a naphtha boiling range petroleum fraction. Such cracked feedstocks should be as free of compounds having fewer than or more than four carbon atoms as possible; therefore, the cracked feedstock may require preliminary processing to remove such light ends and heavy ends. Thus, the cracked feed stream will comprise hydrocarbons having four carbon atoms and, preferably, comprise butene-1, butene-2, isobutylene and butanes, either n-butane or isobutane, or both. The most preferred component of the cracked feed is isobutylene.

While the composition of the cracked feed stream will generally vary depending upon its source and the preliminary processing to remove the aforementioned light ends, heavy ends and a major portion of any diolefins present, Table I below is provided to give the general concentration ranges for the most important compounds of the cracked feed.

                  TABLE I                                                          ______________________________________                                         General Concentration Ranges for Key Components of a                           Cracked Feed in Terms of Volume Percent                                                   Volume %                                                            ______________________________________                                         isobutane    1-40                                                              n-butane     1-15                                                              isobutylene  greater than 5                                                    butene-1     less than about 50                                                butene-2     less than about 50                                                diolefin     up to about 2                                                     ______________________________________                                    

It is most desirable for the feed to a polyisobutylene process unit to have as high a concentration of isobutylene as is possible in order to produce a polyisobutylene product having suitable physical properties. It is also important in the production of a desirable polyisobutylene product for the feed to the polyisobutylene polymerization process unit to have a low butene-1 concentration relative to the isobutylene feed concentration. It is, moreover, important to the operation of a polyisobutylene polymerization process and to the production of a desirable polyisobutylene end-product for the polyisobutylene process unit feedstock to be as free of diolefin contaminants as possible.

Often, the concentrations of the major components of a cracked feed will not be within the ranges required for such a cracked feed to be one suitable as a feed to a polyisobutylene process unit and, thus, the cracked feed must be processed in a manner as to provide a feed stream that is acceptable for a polyisobutylene process unit.

It is a critical aspect of this invention for the feed to the polyisobutylene process unit to, generally, have a concentration of isobutylene of at least about 40 volume percent, preferably at least about 45 volume percent and, most preferably, at least 50 volume percent. Additionally, the molar ratio of butene-1 to isobutylene in the polyisobutylene process feed generally should be less than about 0.30:1, preferably less than about 0.25:1 and, most preferably, less than 0.20:1. It is also important for the polyisobutylene process feed to have a concentration of diolefins as low as possible, which is generally less than about 100 ppm, preferably less than about 50 ppm and, most preferably, less than 10 ppm.

Therefore, the inventive process provides for the processing of a cracked feed, which in itself does not have the required concentrations of components for it to be a suitable polyisobutylene process feed, in order to provide a feed stream to a polyisobutylene process unit having the requisite properties and feed component concentrations. The cracked feed stream is passed to a hydroisomerization unit, whereby diolefins in the cracked feed are selectively hydrogenated to form olefins, and butene-1 is isomerized to butene-2, to produce a hydroisomerate stream.

The catalysts utilized in the hydroisomerization unit of this invention comprise the noble metals of Group VIII of the Periodic Table of Elements, as listed in the Handbook of Chemistry and Physics, published by the Chemical Rubber Company, in the 49th Edition (1969), page B-3. The catalysts intended to be included in the group of noble metals of Group VIII specifically are ruthenium, rhodium, palladium, osmium, iridium, and platinum.

Any of the usual catalyst supports can be employed, such as alumina (preferred), silica alumina, glass beads, and carbon. Catalysts in the form of pellets, spheres, and extrudates are satisfactory.

A preferred hydroisomerization catalyst is palladium on a carrier, the carrier preferably being alumina. The catalyst should contain from about 0.005 to about 2.0 percent palladium on alumina, preferably about 0.1 to about 1.0 weight percent palladium on alumina. Most preferably, the catalyst should contain from about 0.3 to about 0.5 weight percent palladium on alumina. A suitable catalyst weights about 40 to about 60 pounds per cubic foot, has a surface area of about 30 to about 150 square meters per gram, a pore volume of about 0.35 to about 0.50 mL per gram, and a pore diameter of about 200 to about 500 Å.

As an example, a suitable commercial hydroisomerization catalyst satisfactory for use in this invention is manufactured by Mallinckrodt Specialty Chemicals Company, designated as Calsicat catalyst number E-144 SDU. The commercial catalyst contains about 0.55 weight percent palladium on alumina.

The hydroisomerization process is conducted at a reaction temperature of about 100° to about 300° F., preferably 130°-200° F.

The hydroisomerization process of this invention can be most effectively practiced at relatively low pressure conditions while maintaining the hydrocarbon most preferably in the liquid phase, although vapor phase operation can be used. Pressures employed for the liquid phase process are from about 100 to about 600 psig, preferably from about 150 to about 300 psig. Liquid hourly space velocities, LHSV, are: maintained from about 2 to about 50, preferably from about 3 to about 10.

Hydrogen is utilized in the hydroisomerization process by preferably being mixed with the hydrocarbon feed stream prior to contacting the stream with the hydroisomerization catalyst. The hydrogen is necessary to effect double bond isomerization of the 1-olefin with the hydroisomerization catalysts and to provide for hydrogenation of diolefins to olefins. The hydrogen is added in amounts from 0.1 to 20.0 tool percent, preferably in amounts of about 1.0 to about 10.0 tool percent.

The hydroisomerate stream from the hydroisomerization unit, comprising isobutylene, butene-1, butene-2 and at least one paraffin compound, is passed to separation means for separating isobutylene and butene-2 and/or normal butane. The separation means can be any apparatus that suitably provides for the requisite separation. It is preferred for the separation means to be a conventional fractionator column or distillation unit which defines a distillation zone and gives a first overhead stream comprising isobutylene and a first bottoms stream comprising butene-2. The first overhead stream can further contain the compounds of isobutane and butene-1, and the first bottom stream can further contain the compound of n-butane.

The function of the separation means is to separate the isobutane, isobutylene and butene-1 from the butene-2 of the hydroisomerate stream in order to control the feed composition of the feed stream to an isomerization unit.

The first overhead stream is charged or passed to the polyisobutylene polymerization process unit, or polymerization unit, where it is contacted with a polymerization catalyst within a polymerization zone under polymerization conditions. The polymerization temperature generally is in the range of from 50° F. to 325° F.

The catalyst utilized in the polymerization process unit of this invention can include any material that suitably induces the polymerization of isobutylene to form polyisobutylene. Such catalyst materials can include for example, acidic substances, Ziegler-Natta catalysts, and various other initiators.

Many Bronsted acids can initiate the polymerization reaction and include, for example, sulfuric acid, phosphoric acid, dihydroxy fluoroboric acid, and molybdic acid. While such Bronsted acids may induce polymerization, they generally are not preferred because the low molecular weight polymers produced. However, preferred polymerization catalysts include active halide catalysts such as Lewis acids including BF₃, AlCl₃, AlBr₃, TiCl₄, SnC₄ and the like and related electron acceptors such as trialkylaluminums and alkylaluminum halides. Such Lewis acid polymerization catalysts are preferred due to their ability to produce higher molecular weight polyisobutylenes. Certain solid materials that have acidic surfaces can also induce the polymerization of isobutylene and include natural clays, synthetic zeolites, silica-alumina, active carbon, and the reaction product of phosphoric acid with kieselguhr.

The polymer product, or polyisobutylene product, produced by the polymerization unit is separated from the polymerization reaction effluent with the remaining non-reactive hydrocarbons passing from the polymerization unit as an effluent stream for further downstream processing. The isobutylene component of the polymerization unit feed stream is the principal reactive component, thus, the effluent from the isomerization unit will principally contain the non-reactive paraffins, such as isobutane and n-butane, and the lesser reactive or even non-reactive, linear olefins such as butene-1 and butene-2. Therefore, the effluent stream from the polymerization unit will generally contain butanes and linear butylenes; but, more specifically, the effluent stream contains isobutane, n-butane, butene-1 and butene-2.

The effluent stream from the polymerization unit can be mixed with the first bottoms stream of the separation means to form a mixture that is passed as a feed to an extractive distillation unit. The extractive distillation unit is utilized to separate butanes from butenes so as to purge from the overall integrated process system butanes that build up as a result of their non-reactivity in the subprocess units and the recycle streams. Because the close boiling temperatures of butanes and butenes, extractive distillation is required to be used to perform the separation as opposed to other separation methods such as conventional functional distillation.

Extractive distillation is a known separation method and is described in detail in literature such as Perry's Chemical Engineers' Handbook. Sixth Edition, published by McGraw-Hill Company 1984, page 13-53 through 13-57 and U.S. Pat. No. 3,687,202, both of which are incorporated herein by reference.

Any conventional extractive, solvent can be utilized in the extractive distillation unit which permits the separation of the paraffins and olefins of the mixture. Examples of suitable extractive solvent include acetonitrile, dimethylformamide, furfural, acetone, dimethylacetamide, n-methylpyrridone, dimethylsulfoxide, sulfolane, and n-formylmorpholine. These solvents can be used alone or with a cosolvent such as water. The preferred extractive solvents include acetonitrile, n-methylpyrridone and sulfolane. The mixture of polymerization unit effluent and first bottoms stream, which comprises alkanes and alkenes and, more specifically, comprises linear butenes and butanes, is fed to an extractive distillation tower where it is contacted with a solvent. The solvent alters the relative volatilities of the alkanes and alkenes thereby permitting the separation of such compounds into a second overhead stream comprising at least one paraffin compound and a bottoms stream. The bottoms stream from the extractive distillation tower is passed to a stripping tower which provides a third overhead stream comprising at least one olefin. The second overhead stream serves to purge the overall integrated process system of butanes in order to prevent an excessive buildup thereof within the overall integrated process system.

The third overhead stream from the extractive distillation unit is charged or passed to an isomerization process unit for isomerizing at least a portion of the butene-2 of the second bottoms stream to isobutylene and to produce an isomerate stream, which contains the isobutylene formed by way of the isomerization reaction and non-reactive paraffins and lesser reactive or even non-reactive butene-1.

The isomerization process step utilizes a equilibrium type reaction in which butene-2 is isomerized to isobutylene; therefore, for the highest conversion to isobutylene, it is important for the charge to the isomerization unit to be as free of an isobutylene concentration as possible. This is achieved by utilizing as its charge the second bottoms stream that is depleted of isobutylene.

The olefin isomerization process step includes charging or passing the third overhead stream from the extractive distillation unit to the isomerization process unit along with an added water or steam diluent present in an amount of at least about 0.1 mole of water or steam per mole of olefin.

The acidic alumina catalysts utilized in the isomerization process unit are those known in the art. Preferably, the alumina should have a surface area of at least 50 m² /g. In the practice of the present invention, the alumina is used without the incorporation of substantial amounts of inert solids and does not contain substantial amounts of impurities. Good results are obtained with aluminas having a purity of at least about 99.50 weight percent. The alumina can be in any desired form suitable for contact with the olefin including the example granules, spheres, microspheres, pellets, tablets, fluid powder, etc. Preferably alumina catalysts include catalytic beta-alumina and gamma-alumina.

The isomerization catalyst can be employed in any manner conventional within the art, such as in a fixed bed, a fluidized bed and the like.

The isomerization process can be carried out either batch-wise or continuously, using a fixed catalyst bed, stirred batch reactor, a fluidized catalyst chamber, or other suitable contacting techniques. The isomerization process conditions should be suitable to carry out the conversion of the particular olefin involved.

In general, the isomerization process can be carried out at a temperature from 600° to 1200° F., preferably from about 850° F. to about 1000° F. Any convenient pressure can be used, with the lowest practical pressure preferred in order to minimize side reactions such as polymerization. Pressures ranging from atmospheric to 200 psig are particularly suitable.

The LHSV is generally in the range of about 0.1 to 30 volume liquid olefin/volume of catalyst/hr., preferably about 0.2-20.

The isomerate product or isomerate stream from the isomerization unit, which is rich in isobutylene as a result of the isomerization subprocess, is recycled and introduced into the olefin feed stream so that it is charged to the hydroisomerization unit. This recycle stream is responsible for the buildup of paraffin hydrocarbons in the overall process system due to the non-reactivity of the alkanes in the subprocesses of the system.

It is apparent from a study of the overall process that the specific combination and arrangement of process steps is critical to the provision of a feed stream to the polymerization unit having the necessary composition as described earlier herein. In order to better control the polymerization unit feed composition, the separation means can be controlled to provide the desired volumetric ratio of butene-1 to isobutylene in the polymerization unit feed stream. Also, the separation means can be controlled to provide the desired concentration of isobutylene in the feed to the polymerization unit.

Now referring to FIG. 1, there is provided a schematic representation of process system 10 of this invention. A cracked feed stream is introduced by way of line 12 into hydroisomerization unit or process system 14 whereby the cracked feed stream is processed to form a hydroisomerate stream and a light ends stream comprising unreacted hydrogen and light hydrocarbons. Hydrogen is added to the cracked feed by way of conduit 16 prior to introducing the resulting mixture to hydroisomerization unit 14. The light ends stream passes from hydroisomerization unit 14 by way of conduit 18.

The hydroisomerate stream passes from hydroisomerization unit 14 by way of line 20 and is charged or fed to fractionator 22. Fractionator 22 provides for the separation of the hydroisomerate stream into a first overhead stream, comprising isobutylene, and a first bottom stream, comprising butene-2. The first overhead stream passes from fractionator 22 by way of line 24 to polyisobutylene system or unit 26. In polyisobutylene unit 26 the isobutylene of the first overhead stream is polymerized to form a polyisobutylene polymer product. Passing from polymerization unit 26 through line 28 is the polyisobutylene end-product, and through line 30 the effluent from polyisobutylene unit 26 passes and is charged or fed to extractive distillation unit 31. The first bottoms stream from fractionator 22 by-passes polyisobutylene unit 26 by way of line 32 and is mixed with the effluent stream prior to passing the thus-formed mixture to extractive distillation column 33 of extractive distillation unit 31.

Extractive distillation unit 31 is a conventional extractive distillation process which utilizes a suitable extractive solvent to alter the relative volatilities of the alkanes and alkenes in the feed mixture in order to assist their separation. Thus, charged to extractive distillation column 33 through line 34 is the bottoms stream from stripper column 35 containing a suitable extraction solvent. Extractive distillation column 33 provides for a second overhead stream comprising at least one paraffin compound and a second bottoms stream which is fed to stripper column 35 through conduit 36 and is rich in extraction solvent and olefin. Stripper column 35 provides for separation of the second bottoms stream into a third overhead stream comprising at least one olefin and a bottoms stream which is charged to extractive distillation column 33 through line 34.

The second overhead stream passes from extractive distillation unit 31 by way of line 37 and serves to purge the overall process system of unreactive alkanes that enter via the cracked feed in line 12. The third overhead stream, which is enriched with linear olefins, passes from extractive distillation 31 through line 38 to isomerization unit 40 whereby the third overhead stream is processed to form a by-product stream and an isomerate stream. The by-product stream passes from isomerization unit 40 through line 42 and the isomerate stream which is enriched with isoalkenes is recycled through line 44 to be combined with the cracked feed passing through line 12 prior to feeding such combination to hydroisomerization unit 14.

CALCULATED EXAMPLE

To illustrate the inventive process shown in FIG. 1, this calculated example is provided. The material balance for the calculated example is provided in Table II. The stream numbers shown in Table II correspond to those represented in FIG. 1. The material balance provided in Table II assumes a simplified isomerization reactor where the by-product reaction is a disproportionation reaction.

                                      TABLE II                                     __________________________________________________________________________     Material Balance                                                               mass Stream                                                                    fraction                                                                            12   16  18  20   24   28   30   32   37   38   42   44                   __________________________________________________________________________     Hydro-                                                                              0.0000                                                                              1.0000                                                                             0.0062                                                                             0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000               gen                                                                            Propane                                                                             0.0000                                                                              0.0000                                                                             0.0000                                                                             0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.3495                                                                              0.0000               i-Butene                                                                            0.2401                                                                              0.000                                                                              0.2729                                                                             0.2746                                                                              0.5459                                                                              0.0000                                                                              0.0567                                                                              0.1102                                                                              0.0421                                                                              0.1055                                                                              0.0559                                                                              0.2925               1-Butene                                                                            0.5324                                                                              0.0000                                                                             0.0724                                                                             0.0729                                                                              0.1312                                                                              0.0000                                                                              0.2726                                                                              0.0376                                                                              0.0389                                                                              0.0974                                                                              0.0000                                                                              0.2041               2-Butenes                                                                           0.0600                                                                              0.0000                                                                             0.5795                                                                             0.5930                                                                              0.2493                                                                              0.0000                                                                              0.5179                                                                              0.7852                                                                              0.3110                                                                              0.7795                                                                              0.0000                                                                              0.4853               i-Butane                                                                            0.0450                                                                              0.0000                                                                             0.0185                                                                             0.0186                                                                              0.0475                                                                              0.0000                                                                              0.0986                                                                              0.0011                                                                              0.1626                                                                              0.0047                                                                              0.0032                                                                              0.0048               n-Butane                                                                            0.1215                                                                              0.0000                                                                             0.0506                                                                             0.0509                                                                              0.0261                                                                              0.0000                                                                              0.0542                                                                              0.0659                                                                              0.4454                                                                              0.0129                                                                              0.0087                                                                              0.0132               Buta-                                                                               0.0010                                                                              0.0000                                                                             0.0000                                                                             0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000               diene                                                                          Pentane                                                                             0.0000                                                                              0.0000                                                                             0.0000                                                                             0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.5826                                                                              0.0000               Polyiso-                                                                            0.0000                                                                              0.0000                                                                             0.0000                                                                             0.0000                                                                              0.0000                                                                              1.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000                                                                              0.0000               butylene                                                                       Mass 27,557.50                                                                           3.15                                                                               80.98                                                                              80,401.73                                                                           30,325.59                                                                           15,728.30                                                                           14,597.29                                                                           50,076.14                                                                           7,536.14                                                                            57,137.29                                                                           4,215.23                                                                            52,922.06            Flow,                                                                          lb/h                                                                           __________________________________________________________________________ 

That which is claimed is:
 1. A process comprising the steps of:passing an olefin feed stream to a hydroisomerization unit to thereby produce a hydroisomerate stream; passing said hydroisomerate stream to a distillation zone to thereby separate said hydroisomerate stream into a first overhead stream comprising at least about 40 volume percent isobutylene, less than about 100 ppm diolefin, and a molar ratio of butene-1 to isobutylene of less than about 0.30:1 and a first bottoms stream comprising butene-2; passing said first overhead stream to a polymerization unit to thereby produce a polyisobutylene product stream and an effluent stream; mixing said first bottoms stream with said effluent stream to form a mixture; passing said mixture to an extractive distillation unit whereby said mixture undergoes an extractive distillation utilizing a suitable extractive solvent to separate paraffins and olefins by providing a second overhead stream comprising at least one paraffin compound and a third overhead stream comprising at least one olefin compound; passing said third overhead stream to an isomerization unit to thereby produce an isomerate stream; and introducing at least a portion of said isomerate stream into said olefin feed stream.
 2. A process as recited in claim 1 wherein the concentration of isobutylene in said first overhead stream is at least about 45 volume percent of the total volume of said feed stream.
 3. A process as recited in claim 2 wherein the molar ratio of butene-1 to isobutylene in said first overhead stream is less than about 0.25:1.
 4. A process as recited in claim 1 wherein the concentration of isobutylene in said first overhead stream is at least 50 volume percent of the total volume of said feed stream.
 5. A process as recited in claim 4 wherein the molar ratio of butene-1 to isobutylene in said first overhead stream is less than 0.20:1.
 6. A process as recited in claim 5 wherein the concentration of diolefin in said first overhead stream is less than about 50 ppm.
 7. A process as recited in claim 3 wherein the concentration of diolefin in said first overhead stream is less than about 50 ppm. 